1. Field of the Invention
This invention relates to a high-pressure discharge lamp which can be used as a light source in various apparatuses such as a liquid-crystal projector, an illuminator, or a lighting equipment.
2. Description of the Related Art
In a prior-art high-pressure discharge lamp, an arc tends to flicker or fall into disorder when the lamp is operated by electric power having a frequency in the range of 10 to 200 kHz. In a worst case, the arc goes out.
A conceivable cause of that phenomenon is as follows. Some mechanisms in the discharge lamp convert the electric power into acoustic energy (sound energy) having a frequency equal to the frequency of the electric power. Acoustic resonance occurs in the discharge lamp. During the presence of the resonance, an acoustic standing wave (a sound standing wave) is developed in the discharge lamp. The acoustic standing wave makes the arc unstable.
Japanese published unexamined patent application 61-126757 discloses a high-pressure discharge lamp in which soft members made of quartz wool are provided on the inner surfaces of ends of an arc tube. The soft members absorb sound waves, thereby controlling the growing up of a resonant sound wave. Accordingly, an arc in the arc tube is stable in a wide frequency band related to electric power fed to the arc tube.
It is known to operate a high-pressure discharge lamp by rectangular electric wave having a frequency in the range of 100 to 300 Hz. Such low-frequency operation of the discharge lamp provides a stable arc.
In the case where a high-pressure discharge lamp is operated horizontally by low-frequency rectangular electric wave, an arc in the discharge lamp bends or curves upwards. Thus, in this case, the temperature of a top portion of the discharge lamp tends to locally rise. Such a temperature rise causes devitrification in the discharge lamp at an early stage, and hence shortens the life of the discharge lamp.
When a high-pressure discharge lamp having an upwardly curving arc is used in combination with an optical system such as a reflecting mirror, the asymmetry of the arc shape causes the illuminance at an illuminated surface to significantly vary from position to position.